doc: Use default theme
doc: Use default theme

When use Read The Docs to host the documentation,
it seems better to use the default theme of sphinx
rather than using the theme from oslosphinx.
The them from oslosphinx is not rendered as expected.

Change-Id: Ib5a9c34d17349ad02e63b51d88b1fb51688c58b9

openstack/networking-nec,stackforge/networking-nec,stackforge/networking-nec,openstack/networking-nec